Physical stability of colloids

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Chapter: Pharmaceutical Drugs and Dosage: Colloidal dispersions

Physical stability of colloidal dispersions depends on the balance of the following forces:

Physical stability of colloids

Physical stability of colloidal dispersions depends on the balance of the following forces:

1.        Electrical forces of repulsion between dispersed-phase particles

2.        Forces of attraction between dispersed-phase particles

3.        Forces of attraction between the dispersed phase and the dispersion medium

Accordingly, colloidal dispersions can be stabilized by the following:

1. Modulating the electric charge on the dispersed particles. The pres-ence and magnitude, or the absence, of a charge on a colloidal particle are important determinants of the stability of colloidal systems. This can be done through ion adsorption, dissociation of ionizable func-tional groups, and ion dissolution. In addition, ionized species added to the aqueous solution, such as salt, can influence the overall zeta potential on the surface of the dispersed phase.

2. Surface coating of the particles to minimize adherence on collisions. This effect is significant for hydrophilic colloids. Thus, addition of soluble hydrophilic polymers to colloidal dispersions can entangle dispersed-phase particles, minimizing the speed and impact of inter-particle collisions.

The stabilizations strategy depends on the type of colloid and the specific properties of a colloidal system.

Stabilization of hydrophilic colloids

Hydrophilic and association colloids are thermodynamically stable and exist in a true solution, so that the system constitutes a single phase and is visually clear. When negatively and positively charged hydrophilic colloids are mixed, the particles may separate from the dispersion to form a layer rich in the colloidal aggregates. The colloid-rich layer is known as a coac-ervate, and the phenomenon by which macromolecular solutions separate into two liquid layers is referred to coacervation. For example, when the solutions of gelatin and acacia are mixed in a certain proportion, coacer-vation results. Gelatin at a pH below 4.7 (its isoelectric point) is positively charged, whereas acacia carries a negative charge that is relatively unaf-fected by pH in the acid range. The viscosity of the outer layer is markedly decreased below that of the coacervate, which is considered as incompat-ibility. Coacervation need not involve the interaction of charged particles. Coacervation of gelatin may also be brought about by the addition of alco-hol, sodium sulfate, or a macromolecular substance such as starch.

In colloidal dispersions, frequent interparticle collisions due to Brownian movement can destabilize the system. Thus, increase in temperature often compromises the physical stability of these systems.

Stabilization of hydrophobic colloids

In contrast to hydrophilic colloids, lyophobic or hydrophobic colloids are thermodynamically unstable but can be stabilized by imparting electric charge on the dispersed particles, which can prevent aggregation by increas-ing the repulsion between like particles. Addition of a small amount of elec-trolyte to a hydrophobic colloid tends to stabilize the system by imparting a charge to the particles. Addition of excess amount of electrolyte may result in the accumulation of opposite ions and reduce the ζ potential below its critical value, leading to destabilization. The critical potential for finely dispersed oil droplets in water is about 40 mV. This high value indicates relative instability and the need for significant electrostatic charge repul-sion for stabilization. In comparison, the critical ζ potential of colloidal gold is nearly zero, which suggests that the particles require only a minute charge for stabilization.

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